1. Field of the Invention
This invention relates to the production of metal shapes of high integrity whereby superior properties characterize the metal shapes. The invention is particularly concerned with the production of metal shapes utilizing powder metallurgy techniques.
2. Description of the Prior Art
It is well established that powder metallurgy techniques are highly useful for achieving certain advantages in the production of metal shapes. The techniques enable the production of homogeneous metal shapes even where rather complex shapes are involved. In the case of superalloys, for example, a uniform and extremely fine grain structure can be attained, and this grain structure is desirable for achieving certain improved mechanical properties. Furthermore, powder particles of superalloy composition can be consolidated and heat treated to achieve a comparatively larger grain structure whereby more suitable high temperature performance is rendered possible. These capabilities are achieved along with the more conventional advantages of power metallurgy. Specifically, attainment of near net shapes (0.1 inch oversize envelopes) is possible, and this represents cost savings up to about 75 percent over conventional forgings.
One technique available for achieving consolidation of powders is hot isostatic pressing. In such an operation, the powder is located in an autoclave and heated to a temperature sufficient to achieve densification and particle bonding in response to isostatic pressure. Pressure in the order of 15,000 psi is typically applied to the powder, and under such conditions, consolidation of the powder particles is achieved with a minimum of internal voids and other defects when compared with casting operations.
One difficulty encountered in the use of hot isostatic pressing involves the need for some means of encapsulating the powder prior to the application of the isostatic pressure. Thus, the powder is porous in nature and in the absence of some encapsulating means, the gas used for applying pressure would penetrate the powder and thereby equalize pressure internally of the preform so that consolidation could not be achieved. Accordingly, the state of the art uses various means such as formed metal, glass, or ceramic containers to provide the necessary encapsulation of the metal powders. Havel U.S. Pat. No. 3,662,313, Chandhok U.S. Pat. No. 3,700,435, and Loersch U.S. Pat. No. 4,023,966 include such teachings.
These methods of powder consolidation are limited in terms of dimensional control and design flexibility of the final desired shape. For example, containment of powders in formed and welded metal cans is limited in design flexibility, particularly where nonre-entrant angles are concerned. In addition, weldments often provide significant localized strengthening of the can which can subsequently lead to poor reproducibility of the cam movement during hot isostatic processing.
Control of shape distortion is also a problem where ceramic molds, loaded with metal powder, are consolidated wherein metal cans using an intermediate pressure transmitting media. Furthermore, the use of glass containment creates a new set of problems in that the differential thermal expansion between the glass container and metal substrate during heating can result in fracture of the glass container and necessitate specialized handling. Penetration of the glass into the porous metal substrate, insufficient support strength (sagging), and dimensional control are other problems characteristic of glass containment utilization.
Barbaras U.S. Pat. No. 3,455,682, Iler U.S. Pat. No. 3,469,976, Baba U.S. Pat. No. 3,585,261 and Lange U.S. Pat. No. 4,041,123 teach a technique involving the use of a mold or cavity for receiving metal powder. A ram is then utilized to compact the powder through a separate pressure transmitting medium such as thoria or vitreous glass while the combination is being heated. Among other problems, these techniques can result in penetration of the compact by the medium so that subsequent machining is required.
Applicants' copending application teaches improvements in the production of precision metal shapes using powder metallurgy techniques. In accordance with these teachings, the preforms are initially provided with an all-encompassing porous coating after which the preform is subjected to a vacuum whereby the preform is degasified. The coated preform is then heated while the vacuum is being maintained to a temperature sufficient to densify the coating and to render the coating non-porous and pressure-tight. This step is followed by hot isostatic processing wherein the preform is located in a chamber surrounded by a gaseous atmosphere. The pressure in the chamber is elevated, and a temperature employed to the extent sufficient to achieve densification and particle bonding. The product of the operation comprises a consolidated powder compact with a minimum of internal voids and other defects of the type often characteristic of cast products. The products of the process are also not susceptible to penetration problems and other problems associated with the procedures described in the aforementioned patents.